The use of tanks for the storage of liquids has become common. Tanks for the storage of water-immiscible liquids, such as crude oil, offshore operate on the water displacement principle, in accordance with which water is taken into or displaced from the storage tank as the volume of stored oil varies. Such tanks can rest on a seafloor or float in water, and the water can be pumped into and out of the tank as required or the water can be removed from or replaced in the tank by gravity flow with change in stored product volume. Prior art which pertains to this subject are U.S. Pat. Nos. 3,889,477; 3,791,152 and 3,429,128.
The water displacement storage of a liquid such as oil or other hydrocarbon permits location of the tank in water, either in a pit filled with water, or in a lake, ocean or river under conditions which can often provide a natural safety factor. Furthermore, utilization of the water displacement principle for product storage provides inherent tank stability since it is always full or nearly full of a liquid, or two liquids in layers. Also, the liquid contents in the tank counter the external water pressure applied to the tank, thus making it feasible to use a thinner tank shell with less reinforcement than would be required for a tank intended at times to be periodically empty, or partially empty but having the water pressure applied to the shell exterior.
In addition to the storage of materials which are liquids at ambient conditions (i.e. , 25.degree. C. and 760 mm. atm. pressure), it is common to storage gases such as natural gas, propane, oxygen and hydrogen in liquefied form, very often at temperatures low enough to permit storage of the liquefied gases at or slightly above atmospheric pressure to make unnecessary the use of a pressure vessel for storage of the liquefied gas. U.S. Pat. Nos. 3,559,835 and 3,419,174 show the storage of liquefied gases in insulated tanks onshore. Others have recognized the need for offshore storage tanks for liquefied gases, and for such tanks onshore but located in water-containing pits. See, for example, U.S. Pat. Nos. 3,898,846 and 3,828,565.
Another U.S. Pat. No. 3,727,418 discloses subaqueous storage of liquefied gases. A first closed container holds a liquefield gas which is separated from a balancing liquid, such as isopentane, therein by a flexible menbrane. A second container communicates with the balancing liquid in the first container so that when a liquefield gas is fed to the first container the balancing liquid flows to the second container and this is reversed when liquefied gas is removed from the first container. The second container is divided by a flexible membrane with balancing liquid on one side and water on the other side. Water flows in and out of the second container by gravity and provides the pressure which forces balancing liquid into the first container. At no time is water introduced into the first container so that a very large amount of isopentane, equal to the first container storage volume, is needed in this system.
U.S. Pat. No. 3,438,215 shows an insulated storage tank with a floating roof which has liquefied methane above, and isopentane below, the roof. The isopentane is used as a cold-carrier. No water is in the tank.